1. Field of the Invention
The present invention relates to a material forming a turbine casing and a valve casing of a steam turbine into which high-temperature, high-pressure steam flows as a working fluid, and in particular, to a nickel-base casting superalloy for steam turbine superior in high-temperature strength and so on, and to a cast component for steam turbine using the same as its material.
2. Description of the Related Art
In a thermal power plant including a steam turbine, an art to reduce carbon dioxide emission has been drawing attention in view of global environmental protection, and a need for highly efficient power generation has been increasing.
For higher efficiency of the power generation of a steam turbine, it is effective to increase the temperature of turbine steam. Recently, a thermal power plant including a steam turbine uses the steam whose temperature is equal to or higher than 600° C. The future trend is toward a higher steam temperature up to 650° C., further 700° C. or over 700° C.
A turbine casing and a valve casing of a steam turbine into which high-temperature, high-pressure steam flows as a working fluid can be regarded as a kind of a high-temperature pressure vessel receiving a high inner pressure under a high-temperature environment. Therefore, the turbine casing and the valve casing are required to endure high temperature and high stress, which is creating a demand for materials having excellent strength, ductility, and toughness at a high temperature range as materials forming the turbine casing and the valve casing.
Further, the materials need to have excellent steam oxidation resistance because of long use at high temperatures. Further, because of their complicated shapes, the turbine casing and the valve casing are generally molded by casting, and therefore good castability is required so that the occurrence of defects at the time of the casting is prevented as much as possible.
Good weldability is also an important factor as the materials because, in the event of the occurrence of a casting defect, repair-welding is necessary after a defective portion is chipped off and because short pipes, elbow pipes, and so on are joined to the turbine casing and the valve casing by welding (structural welding).
Further, in an inner structure of the turbine casing and the valve casing, they are structurally combined with other components when used. For example, inside the turbine casing, a turbine rotor rotating by steam, rotor blades, nozzles (stator blades), tie bolts, nozzle boxes, and so on are assembled. Therefore, in order to facilitate the structure designing and realize greater reliability over a long period of operation, the turbine casing preferably has the same level of thermal expansion coefficient as those of these inner structure components. Further, as the thermal expansion coefficient is lower, a local heat stress as a large structure is smaller, and from this point of view, easy structure designing and improved long-term reliability are realized.
Therefore, a Ni-base casting superalloy used for a turbine casing and a valve casing is required to have excellent strength (creep rupture strength) and ductility (creep rupture elongation) at high temperatures, excellent steam oxidation resistance, excellent weldability, and a low thermal expansion coefficient.
At present, typical materials as a Ni-base casting superalloy whose use in the application where the steam temperature is 700° C. or over 700° is under consideration are an Inconel 617 superalloy (manufactured by Special Metals Corporation) and an Inconel 625 superalloy (manufactured by Special Metals Corporation). However, though satisfactory in creep rupture elongation, steam oxidation resistance, and weldability, these materials do not have sufficient creep rupture strength and have a relatively large thermal expansion coefficient, which leads to a difficulty in the structure designing of the turbine casing and the valve casing using these materials and involves many problems in terms of long-term stable operation of these turbine casing and valve casing at high temperatures.
Conventionally, the use of an austenitic Ni-base superalloy having a low thermal expansion coefficient for bolts and so on of a steam turbine has been proposed (see, for example, JP-A 2003-13161 (KOKAI)). Further, the use of a NiFe-base superalloy from which it is relatively easy to fabricate a large forged product, in the manufacture of a steam turbine rotor used at high temperatures over 700° C. has been proposed (see, for example, JP-A 2005-2929 (KOKAI)). Another proposal is to use an austenitic Ni-base superalloy having good forgeability and a low thermal expansion coefficient in order to manufacture a steam turbine blade (see, for example, Japanese Patent No. 3559681). However, these arts give no consideration to castability, weldability, and the like.